This invention refers to a projection objective with variable focal length, also generally referred to as variable focus lenses, having a front member with negative refractive power and a multiple-lens basic objective with positive refractive power, between which a multiple-lens variator with positive refractive power, displaceable in the direction of the optical axis, is disposed, with the middle lens thereof having a negative refractive power.
A variety of types of variable focus lenses for slide and film projectors have become known. In general, they are derived from the essentially more complexly constructed and complicated zoom lenses for shooting cameras, which are optimized above all in favor of the image performance and are therefore correspondingly expensive. As opposed to this, up to now, variable focus lenses for projection purposes have usually been designed as simply as possible from the point of view of economics of their construction and production.
For example, in the printed German specification DE-AS No. 20 36 285, a variable focus objective consisting of only five lenses has become known, which is equipped with a minimum of lenses for economic reasons. Therefore, the variator of this previously known variable focus objective, like the front member as well, consists of only one single lens. However, such a variable focus objective does not by any means meet the demands made nowadays with regard to freedom from color defects and distortion.
A nine-lens variable focus objective of the type in question, having a variator consisting of two lenses cemented together, has become known in the German laying-open specification No. 26 14 267. Such an objective, which has a complex and costly construction not least of all due to its five-lens basic objective, distinguishes itself by improved chromatic aberration over a variable focus objective with a variator consisting of only one single lens; however, the distortion herein still reaches a relatively high level. A further disadvantage of this previously known variable focus objective consists in the use of two cemented surfaces, which requires a high production expenditure, whereby the objective becomes substantially more expensive.
A further variable focus objective of the type in question, with a two-lens variator, a front member designed as a single lens and a basic objective consisting of a Cookes or Taylor lens has become known in the German laying-open specification No. 26 26 696. In this six-lens variable focus objective, all lenses are separated from each other by air spaces and the two lenses of the variator consist of biconvex, preferably identical single lenses. In order to lower production costs, in this objective the front lens and the variator consist of plastic rather than optical glass. Therefore, it is no wonder that this known objective shows clearly visible distortions, especially in the wide-angle and telescopic settings.
Correspondingly costly variable focus objectives with thirteen, sixteen and fourteen lenses and a large number of cemented surfaces or with ten lenses and two cemented surfaces are known in the German Pat. No. 30 26 931 and the East German Pat. No. 51 130. The distortion in most of these known variable focus objectives for projection purposes shows cushion-shaped lines in the wide-angle setting and barrelshaped lines in the telescopic setting. The values for the distortions definitely amount to up to .+-.5% and are clearly visible during the projection. Such distortions are perceived to be disturbing, above all in fade-over projection, parallel projection or multivision.